Hello,
Question 1
In reference to the W32.Downadup and W32.Downadup.B, I would request you to check these Articles:
Best Practice for Downadup.B and Additional information on the same.
https://www-secure.symantec.com/connect/articles/best-practice-downadupb-and-additional-information-same
Simple steps to protect yourself from the Conficker Worm
http://service1.symantec.com/support/ent-security.nsf/docid/2009033012483648
Today, lot of Threats are created with a Target and Better planned and using the Vulerabilities of the OS and other softwares.
Often, these are not detected by Virus Definitions and that is where you require features like ProActive Threat Protection and Network Threat Protection.
These Features are recommended to provided you complete Endpoint Protection.
Question 2:
In reference to the W32.SillyFDC.BDP and SillyFDC.BDP!LNK
I would recommend you to follow this Plan of Action:
1) Install ALL Latest Microsoft Secuirty Patches / Sevice Packs on ALL machines
2) Disable Auto play with GPO
http://support.microsoft.com/kb/953252
3) Disable Scheduled Tasks with GPO
http://support.microsoft.com/kb/310208
4) Enable Security Auditing with GPO
http://support.microsoft.com/kb/300549
For W32.SillyFDC.BDP and SillyFDC.BDP!LNK, here are the Symantec Writeup's:
http://www.symantec.com/security_response/writeup.jsp?docid=2011-031106-4835-99
http://www.symantec.com/security_response/writeup.jsp?docid=2011-053115-1305-99
For Question 3:
"Is it possible a host which have been patched with .LNK & Downadup patch and without shared folder being infected by SillyFDC.BDP/BDP!LNK ?
This host has no AV installed though...."
I believe you have answered you own Question.